1. Field of the Invention
The present invention relates to a recording unit, an image recording apparatus and an image recording method.
2. Related Background Art
The ink jet recording method is a non-impact recording method of low noise, capable of recording an image by directly discharging ink onto a recording medium. This method, not requiring a complex apparatus in exploitation, is capable of easily achieving a low running cost, compactization of apparatus, color image recording etc.
For this reason, the ink jet recording method has been commercialized in various recording apparatus such as a printer, a copying apparatus, a facsimile apparatus and a word processor. Also such ink jet recording technology has been applied to a color ink jet recording apparatus for forming a multi-color image utilizing black ink and color ink (for example at least a color ink selected from yellow ink, cyan ink, magenta ink, red ink, green ink and blue ink).
On the other hand, the ink jet recording method is associated with a drawback of so-called bleeding phenomenon that, in case two different inks are deposited in adjacent manner on a recording medium, such inks are mutually mixed at the boundary to deteriorate the quality of the recorded image. In particular, the color mixing at the boundary of black ink and color ink has a strong influence in deteriorating the image quality, and various methods for resolving such drawback are therefore being developed.
A representative resolving method consists of an ink set and a recording method utilizing the same, which incorporates a mechanism, in case two inks are deposited in mutually adjacent manner onto a recording medium, of causing a viscosity increase in at least one of the two inks, or coagulation or precipitation of the coloring material of at least one of the two inks, thereby preventing the bleeding phenomenon.
For example, U.S. Pat. No. 5,428,383 discloses incorporating a precipitating agent (for example a polyvalent metal salt) in one of the inks and employing a coloring agent consisting of an organic dye having a carboxyl radical and/or a carboxylic acid salt radical in the other of the inks, preferably in the black ink. When these inks are printed in a mutually adjacent manner, the first ink containing the precipitating agent causes the coloring agent having the carboxyl radical and/or the carboxylic acid salt radical to precipitate, whereby the coloring agent is prevented from moving to the other ink, thereby reducing the bleeding phenomenon between the two adjacent print areas.
Also the U.S. Pat. No. 5,976,230 discloses a technology of depositing two mutually reacting inks in a same area thereby preventing the bleeding phenomenon.
There has been investigated the configuration of the ink jet head more advantageously using an ink set including the combination of such black ink and color ink. As a result, it is concluded that, in the case of employing a polyvalent metal salt as a component for providing reactivity between the two different inks, the configuration of a portion in the liquid flow path of the ink jet head coming into contact with the ink, particularly a portion above the heat-generating member (electrothermal converting element), for example the configuration of the protective layer, plays an important role in improving the discharge durability of the ink jet head. In such configuration, it is concluded that a discharge system, in which a bubble generated by the thermal energy provided by the heat generating member at the discharge of an ink droplet from a discharge port communicates with or is opened through to the external air, is effective for improving the discharge durability even in case of employing an ink containing a polyvalent metal salt.
The present invention has been attained on the above-mentioned finding of the present inventors.
The object of the present invention is to provide a recording unit and an image recording apparatus capable, even in case of employing an ink containing a polyvalent metal salt, of improving the discharge durability while retaining the advantage of utilizing such polyvalent metal salt, and an image recording method utilizing such unit and apparatus.
According to the present invention, there is provided a recording unit comprising an ink container portion for containing an ink which contains a coloring material and an aqueous medium and an ink jet head portion for discharging a droplet of such ink;
wherein the ink is a polyvalent metal salt containing ink which further contains a polyvalent metal salt;
the aforementioned ink jet head portion for discharging the polyvalent metal salt containing ink includes a discharge port for discharging ink, a liquid flow path communicating with the discharge port and supplying the discharge port with the ink; and an electrothermal converting element provided on a bottom face opposed to the discharge port in the liquid flow path and serving to generate thermal energy to be utilized for ink discharge; and
the ink jet head portion functions according to an ink discharge method including a discharge step of discharging an ink droplet from the discharge port utilizing the pressure of a bubble generated in the ink by providing of the thermal energy to the ink present in the flow path, and an ink replenishing step of replenishing ink in a portion for forming the aforementioned bubble after the bubble reaches an open state through to the external air via the discharge port along with the ink discharge from the discharge port.
According to the present invention, there is also provided an image recording apparatus comprising an ink jet head for discharging ink containing a coloring material and an aqueous medium;
wherein the ink is a polyvalent metal salt containing ink which further contains a polyvalent metal salt;
the ink jet head for discharging the polyvalent metal salt containing ink includes a discharge port for discharging the ink, a liquid flow path communicating with the discharge port and supplying the discharge port with the ink; and an electrothermal converting element provided on a bottom face opposed to the discharge port in the liquid flow path and serving to generate thermal energy to be utilized for ink discharge; and
the ink jet head functions according to an ink discharge method including a discharge step of discharging an ink droplet from the discharge port utilizing the pressure of a bubble generated in the ink by providing the thermal energy to the ink present in the flow path, and an ink replenishing step of replenishing ink in a portion for forming the aforementioned bubble after the bubble reaches an opened state through to the external air via the discharge port along with the ink discharge from the discharge port.
According to the present invention, there is further provided an image recording method for forming an image by discharging ink containing a coloring material and an aqueous medium onto a recording medium;
wherein the ink is a polyvalent metal salt containing ink which further contains a polyvalent metal salt;
the ink jet head for discharging the polyvalent metal salt containing ink includes a discharge port for discharging the ink, a liquid flow path communicating with the discharge port and supplying the discharge port with the ink; and an electrothermal converting element provided on a bottom face opposed to the discharge port in the liquid flow path and serving to generate thermal energy to be utilized for ink discharge; and
the ink discharge by the ink jet head includes a discharge step of discharging an ink droplet from the discharge port utilizing the pressure of a bubble generated in the ink by providing of the thermal energy to the ink present in the flow path, and an ink replenishing step of replenishing ink in a portion for forming the aforementioned bubble after the bubble reaches an opened state through to the external air via the discharge port along with the ink discharge from the discharge port.
The present invention adopts, in the ink jet head for discharging the ink containing the polyvalent metal salt, a discharge method in which the bubble generated in the liquid flow path is opened through to the external air via the discharge port, thereby enabling to improve the discharge durability of the ink jet head even in case of utilizing the polyvalent metal salt containing ink.
In an ink jet head of a discharge method in which the bubble is not opened through to the external air, durability is required against a cavitation phenomenon generated at the ink discharge, namely a physical impact resulting from the formation and vanishing of the bubble, in a protective layer provided at least in an ink-contacting portion of a heater surface of the electrothermal converting element. Such cavitation is considered to result from a process that the bubble generated and showing a volume increase in the ink by the heat from the heater assumes a negative internal pressure after the termination of heating by the heater, thereby vanishing after a rapid volume decrease. On the other hand, in the aforementioned discharge method in which the bubble is opened through to the external air, the aforementioned process of negative pressure generation does not take place because the bubble is opened through to the external air, so that the cavitation phenomenon is practically absent.
In case of employing the ink containing polyvalent metal salt in the discharge method in which the bubble is not opened through to the external air, the influence of cavitation, for example the proceeding of corrosion, may be enhanced under the presence of the polyvalent metal salt, and it is required to provide the heater portion with a protective layer of a configuration capable of withstanding such situation. On the other hand, in the ink jet head employing the ink containing the polyvalent metal salt, the use of the discharge method in which the bubble is opened through to the external air allows to relax the requirement on the durability on the protective layer provided on the heater surface. As a result, if the protective layer of a same configuration is applied to the ink jet heads for discharging the polyvalent metal salt containing ink respectively with the discharge method in which the bubble is opened through to the external air and with the discharge method in which the bubble is not opened through to the external air, the former head causes less breakage thereby becoming superior in the discharge durability.